Fix problems noticed by Allan Steel, <allan@maths.su.oz.au>,

Fix problems noticed by Allan Steel, <allan@maths.su.oz.au>,
when debugging a program with 100 shared libraries.

* solib.c (solib_map_sections):  Always close the BFD we open.
Free all malloc'd storage we allocate, too, including error cases.
(struct so_list):  Remove unused so_bfd member.
(clear_solib):  Don't bother closing so_bfd.

* symfile.c (symfile_bfd_open):  Mark newly opened BFD as
cacheable.  `So many symbol files, so little file descriptors.'